In a microwave experiment simulating laser-plasma interactions, the production of fast electrons by the parametric decay instability is found to be significantly decreased by replacing a monochromatic pump with a noisy one. With this experimental arrangement, it is observed that finite bandwidth also inhibits the formation of density cavitons. We have experimentally presented the effects of finite-bandwidth pumps on the parametric instability at power levels sufficient to produce hot electrons.
The experiments were performed in an unmagnetized electrostatically confined filament-discharge plasma of 100 cm length and 35 cm diameter with argon fill pressures of 0.4 mTorr.
The plasma density as a function of the distance from the dielectric
window located at the narrow (entrance) end of the microwave horn
is displayed in the inset of Fig. 1. The mean density of the pulsed
plasma changes by less than 0.1% over the rf pulse duration. The
corresponding change in plasma frequency is much less than the
observed instability resonance width.
At moderate rf power levels (P>7 W and w0~wp) the low-frequency
density fluctuations and high-frequency electric field fluctuations
characteristic of the parametric decay instability are observed.
The instability occurs within the uniform density region located
10-15 cm from the input window (see inset Fig.1). Upon instability
onset, as detected by shielded Langmuir probes of both the double
and single types, an increase in the hot-electron current to the
analyzers is observed. The energetic electrons are found to be
preferentially directed along the electric field of the incident
microwave radiation. Varying the frequency of the narrow-band
pump, we found that the resonance width for hot-electron production
increases from 2.4% of the pump frequency near threshold power
to 5% at higher powers (P~300 W). The effect that the pump
bandwidth can have a significant effect on hot-electron production
when pump bandwidth is larger than resonance bandwidth was investigated
in the present experiment
using both noise-phase-modulated pumps with bandwidths
variable up to 1.2% and noise-amplitude-modulated pumps with bandwidths
up to 23%.
The saturated hot-electron flux is shown in Fig. 2 for both the narrow-band and 300 MHz (Dw/w0 ~10%) noise-amplitude-modulated pumps as a function of pump center frequency for several power levels. Near the instability resonance center frequency, the finite bandwidth pump is observed to reduce or even eliminate the analyzer hot-electron current. However, at high power, the effective instability resonance width is seen to increase for the noise pump. In the case of the broadband pump, hot electrons are detected at pump center frequencies where non are observed with the narrow pump. This phenomenon may partially explain the increase in the instability amplitude observed by Yamanaka et al. in a finite-bandwidth laser heating experiment.
The effect of finite bandwidth on the instability-produced hot-electron
distribution function was investigated. For our plasma, the high-power
narrow-band pumps heat a fraction (<10%) of the electrons in
the 20 eV tail of the initial electron distribution function to
a higher-temperature (~30 eV) Maxwellian velocity distribution.
Similarly, we found that the finite-bandwidth pumps simply decreased
the maximum number of heated electrons, without changing the temperature
of the heated electrons. The relatively small percentage of heated
electrons (<0.1% of the total) may reflect the small size of
the instability region in comparison to the hot-electron mean
Fig. 3 displays the time evolution of the hot-electron current
detected by an energy analyzer for both the narrow and noise-modulated
(Dw/w0~10%) pumps at several
power levels. At low power
levels (P<~21 W ~ 3 Pth), there were no detectable hot electrons for
the wide-band pump, while electron tail heating was observed down to 7 W
with the narrow pump. At higher powers, P>~21 W) hot electrons are produced
by both pumps and we
can compare their respective rates for hot-electron production.
The initial fast growth in the hot-electron current which saturates
and begins to decay ~ 1 usec after turn-on for the narrow
pump [Fig. 3(a)] may be due to cavitation or resonance absorption
arising from the plasma density gradient along the pump electric
field. Finite pump bandwidth appears to drastically inhibit this
early quickly saturated hot-electron flux produced by the wide-band
pump during the first few microseconds is comparable to or larger
than that of the narrow pump. When the pump center frequency is
not adjusted to narrow-band threshold minimum, the wide-band pumps
can have consistently faster growth rates for a given power than
the narrow-band pumps, similar to the saturation results of Fig.
The plasma density disturbances associated with the initial fast-electron production mentioned above were investigated using short rf pulses of variable duration (20-500 nsec). In Fig. 4(a), we show the electron density perturbation, as detected by a Langmuir probe biased to electron saturation, which occurred upon irradiating the plasma with a 100 nsec narrow-band rf pulse at high power. Similar density perturbations, but with reduced amplitude, were observed for rf pulses as short as 40 nsec. Fig. 4(b) shows the result of employing a longer narrow-band pulse with the same average power, pulse duration, and center frequency as the narrow-band pump in Fig. 4(b). The bursts of hot electrons which were observed to occur along the pump field with the short narrow-band rf pulses were also nearly eliminated with the wide-band rf.
In conclusion, we find that finite pump bandwidth can significantly increase the minimum threshold and reduce the saturation level of parametric decay-produced suprathermal electrons. It also can apparently control the hot-electron production due to cavitation. The fact that finite bandwidth did not produce a marked decrease in heating rates for nonthermal electrons on all time scales is somewhat surprising.